An electrolytic capacitor uses, as an anode, a base metal, such as aluminum or tantalum, with its surface oxidized by, for example, anodizing to form an insulating oxidized film as a dielectric layer. This anode faces a cathode with a separator supporting an electrolytic solution therebetween.
The anode usually has an etched surface to increase its surface area. The electrolytic solution that is in close proximity with the etched uneven surface of the anode functions essentially as a cathode. Properties of the electrolytic solution, such as conductivity and temperature characteristics, are therefore decisive on the electrical characteristics of an electrolytic capacitor. Further, since the electrolytic solution serves to restore a deteriorated or damaged insulating oxidized film, it has influences on the leakage current or durability of the electrolytic capacitor. Thus, an electrolytic solution is an important element in determining the characteristics of an electrolytic capacitor.
Of the various characteristics of an electrolytic solution, conductivity is directly related to a loss, an impedance characteristic, etc. of an electrolytic capacitor. From this standpoint, intensive study has recently been directed to development of an electrolytic solution having high conductivity. Among the electrolytic solutions proposed to date, those comprising a quaternary ammonium salt of an organic acid, especially a carboxylic acid, as a solute, and an aprotic solvent, e.g., .gamma.-butyrolactone, are noteworthy for their high conductivity. Electrolytic solutions of this type are described, e.g., in JP-B-3-6646 and JP-B-3-8092 (the term "JP-B" as used herein means an "examined published Japanese patent application").
Such a highly conductive electrolytic solution generally has a low dielectric strength in itself and has been used in a region of a rated voltage of 50 V or less. For use in a region having a rated voltage exceeding 50 V, dielectric strength may be increased by a combined use with an electrolytic solution suitable for use in a high-voltage region, such as a boric acid-ethylene glycol system. Such a combined use, however, unavoidably results in a considerable reduction in conductivity because of low conductivity of the electrolytic solution for a high-voltage region. It is possible to prevent conductivity reduction by addition of water, but such a manipulation is known to adversely affect the upper working limit of temperature and the working life of the electrolytic solution and is not therefore recommended.
It has also been proposed to add to a highly conductive electrolytic solution a chemical effective on improvement of dielectric strength. Among this approach is addition of an alkyl phosphate, as disclosed, e.g., in JP-A-3-209810 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Further, addition of a dispersion of colloidal silicon dioxide has been proved effective to improve dielectric strength as disclosed in JP-A-4-58512.
These conventional approaches for increasing dielectric strength of a highly conductive electrolytic solution each have disadvantages. That is, a combination of an otherwise highly conductive electrolytic solution with an electrolytic solution for a high-voltage region involves a great reduction in conductivity as mentioned above and fails to satisfy both conductivity and dielectric strength requirements. In the case of using various additives, some additives improve dielectric strength only to an insufficient extent; or an increase in dielectric strength is limited to a certain level, and no further improvement is obtained even with an increased amount of the additive; or some additives adversely affect conductivity. Further, some additives seemingly attain an increased dielectric strength, but the increased level of dielectric strength is too unstable and unreliable and cannot be maintained in the final product solution.